Pulse system



Jan. 1l, 1949. H QYPETERSON l 2,458,633

PULSE ssTEM A Filed Jan. 5, 1944 /9 20 2/ es 24 25 26 27v 2a' 29 y/ Tlc. I

y MARE( I ATTORNEY Patented Jan. 11, 1949 PULSE SYSTEM Harold O. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 5, 1944, Serial No. 517,002

3 Claims.

The present invention comprises a pulse communication on-off keying system utilizing pulses which are always substantially 50% mark or weight and in which energy is radiated during only the mark intervals. I have found that a pulse system utilizing pulses of substantially 50% mark requires the narrowest receiver band width of any pulse communication system. The term 50% mark is herein used to refer to that condition where the mark pulses have the same duration as the space intervals.

The following is a detailed description of the invention accompanied bya drawing, wherein:

Fig. 1 diagrammatically illustrates a transmitter for producing pulses of 50% mark in accordance with the invention;

Fig. 2 diagrammatically illustrates a receiver for use in connection with the transmitter of Fig. 1;

Figs. 3, 4 and 5 illustrate the wave forms to be found in certain connections of the receiver of Fig. 2; and Y Fig. 6 graphically illustrates the effect that pulses of different mark or weight have on the output amplitude of the receiver.

Referring to Fig. 1 in more detail, there is shown a transmitter for always obtaining pulses of substantially 50% mark. This transmitter comprises a reactance tube I for varying the frequency of an oscillator II, in accordance with signal modulations from a microphone I 3, by way of example. The oscillator II may generate, by way of example, oscillations of 100 kilocycles. The output of oscillator II is coupled to a frequency converter I4 where the oscillations beat with oscillations from a heterodyne oscillator I5 to produce lower frequency waves in the output of the converter. The beating oscillator I5 may generate oscillations of about 110 kilocycles. Thus, the output of the converter I4 which is impressed upon the square wave generator I6 will be a variable frequency sinusoidal wave of about ten kilocycles. This ten kilocycle wave, which varies in frequency in dependence upon the signal modulation, causes the square wave generator I6 to produce square wave pulses of substantially 50% mark. The square wave generator may be of the flip-flop variety. Alternatively, if desired, the output of the converter I4 can be impressed upon a limiting amplifier in order to obtain a square wave which is 50% mark. One such limiting amplier which can be used is described in Crosby United States Patent 2,276,565, granted March 17, 1942. These square wave pulses are impressed upon the input of a radio frequency 2 generator I'I to cause the generator I7 to produce correspondingly spaced pulses of radio frequency energy which are then passed on to an antenna I8 where the radio frequency waves are radiated to a distant receiver. The mark pulses are separated by space intervals during which no energy is radiated. In practicing the invention, the pulse rate should always be greater than twice the highest modulating frequency.

Fig. 2 shows a receiving system for receiving'` the 50% mark pulses transmitted by the system of Fig. 1. This receiver comprises an antenna I9 which feeds a radio vfrequency amplifier 20, the latter in turn supplying the amplified pulses to a frequency converter 2I, where the radio frequency energy is beat with the oscillations from a local oscillator 22 to produce pulses of intermediate frequency energy.v The intermediate frequency energy from the output of the frequency converter 2I is supplied to an intermediate frequency amplifier 23 which in turn feeds the pulsesv to a rectifier 2d. Both the intermediate frequency ampliiier 23 and the radio frequency amplifier 20 comprise suitable selectivity and gain circuits. The output of the rectier 24 consists of unidirectional pulses which are optionally amplied at 25 and then passed on to a clipper 26. The rectified or unidirectional pulses supplied to the clipper are of substantially 50% mark and contain noise ripples. The wave form of these rectified pulses is shown in Fig. 3. The clipper 26 removes a portion of the rectied pulse be tween the top and bottom to provide a clean pulse having the noise ripples removed. This portion of the pulse removed from the unidirectional pulse appears between the two horizontal lines labeled Clipper Levels. It will thus be seen that the clipper is a top and bottom limiter which removes noise and variations above and below certain limits. Putting it in other Words, the clipper removes the noise from the flat tops of the direct current pulses and also the noise from the spaces between the pulses. The clipped pulses from the output of 2 0 may have the wave form of Fig. 4. It willbe seen that the amplitudes of the clipped pulses are constant although the lengths of the pulses and the phase may have variations caused by noise. The bi-directional arrows on the starting and trailing edges of the pulses of Fig. 4 are intended to designate variations in length and phase caused by noise.

The output of the clipper is fed into a band pass filter 21 which is designed to pass the mean pulse frequency plus and minus sideband frequencies produced by the signal modulation.

The output of the band pass filter will have a Wave form, as shown substantially by Fig. 5. The carrier in the output of filter 21 will have a variable frequency and can then be supplied to a suitable frequency modulation discriminator and detector circuit 28, where the variable frequency is converted to amplitude modulations. The resultant amplitude modulated wave can then be fed to an audio frequency amplifier 29 whose output is coupled to a suitable acoustic transducer 30. This acoustic transducer is shown to be a pair of headphones, although it will be understood that it can be a loudspeaker or any suitable recording device.

Although it has been shown that noise has the effect of varying the percent mark of the received pulses, particularly as they come out of the clipper, these variations in the percent mark at values near 50% have little or no effect on the output amplitude. This is illustrated in Fig. 6, which graphically shows the amplitude of the wave in the output of the band pass filter 2'1 plotted as current output on the ordinate scale and percent mark on the abscissa scale. The system thus enables one to obtain a very large gain in signal to noise ratio, provided the pulse amplitude is above the threshold value of about twice the peak noise voltage. Also, by means of experiments, I hav-e found that pulses of 50% mark require the narrowest receiver band width of any pulse system.

The present invention is designed particularly for use in an on-off keying system, in which energy is radiated during the on or mark intervals and no energy is radiated during the "oi or space intervals.

What is claimed is:

1. The method of operating a transmitter in a pulse communication system which comprises varying the frequency of an oscillator in accordance with signal modulations, beating said frequency with constant frequency oscillations to produce a lower varying frequency, controlling a square wave generator by said lower Varyingfrequency to produce spaced pulses each of which has substantially the same duration as the intervals between them, and generating pulses of radio frequency energy under control of said aforementioned spaced pulses.

2. A pulse transmitter for sending spaced pulses each of which has substantially the same duration as the intervals between them, comprising an oscillation generator, a reactance tube modulator coupled to said oscillator for varying the frequency or phase of the oscillations from said oscillator in accordance with signal modulations, a frequency converter, a constant frequency oscillator generating oscillations different than said rst oscillator, connections from both oscillators to said converter to thereby produce a variable beat frequency of lower frequency, a square wave generator coupled to the output of said converter and producing spaced pulses whose durations are always substantially equal to the intervals between the pulses, and a radio frequency oscillator under control of the output of said square wave generator.

3. The-method of communication which comprises producing spaced pulses whose durations are always substantially equal to the intervals between them, at a rate greater than twice the highest modulation frequency, and varying the occurrence time of said pulses in accordance with signal modulations without substantially varying the durations of said spaced pulses.

HAROLD O. PETERSON.

' REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,113,214 Luck Apr. 5, 1938 2,266,401 Reeves Dec. 16, 1941 2,289,041 Van B. Roberts July '7, 1942 2,292,100 Bliss Aug. 4, 1942 2,392,546 Peterson Jan. 8, 1946 y FOREIGN PATENTS Number Country Date 541.665 Great Britain Dec. 5. 1941 

